Evanescent hemolysis detection

Abstract

Analyte content in a cell free portion of a body fluid, such as blood, is optically determined without centrifugation or other preliminary steps for separating the cell free portion from the body fluid. A channel is configured for containing a flowing sample of the body fluid along an optical boundary. The channel is configured so that a cell free layer of the fluid naturally forms along the boundary of the channel which coincides with the optical boundary. A light source is directed onto the optical boundary at an angle selected to generate total reflection from the boundary and to generate an evanescent field across the boundary in the cell free layer of fluid. A light detector is configured to detect absorption of the light in the evanescent field. The light source and light detector are matched to the wavelength range of an absorption peak of the analyte being detected.

What is claimed is: 
     
       1. A method for detecting analytes in a flowing whole blood sample, the method comprising:
 directing a first light source through an optically transmissive medium to an optical interface between the optically transmissive medium and the flowing whole blood sample, wherein the optically transmissive medium is glass and wherein light from the first light source is incident at the optical interface at an angle of incidence greater than or equal to a critical angle relative to a normal of the optical interface, wherein the angle of incidence creates total internal reflection of the light from the first light source and creates an evanescent field extending into the flowing blood sample, wherein the evanescent field extends into a plasma layer of the flowing whole blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the flowing whole blood sample containing blood cells; 
 measuring absorption of the light from the first light source by an analyte in only a plasma layer of the flowing whole blood sample within the evanescent field; 
 directing a second light source through optically transmissive medium to the optical interface, wherein light from the second light source has a wavelength that differs from a wavelength of the light from the first light source and wherein the light from the second light source is incident at the optical interface at second an angle of incidence greater than or equal to the critical angle relative to the normal of the optical interface, wherein the second angle of incidence creates total internal reflection of the light from the second light source and creates a second evanescent field extending into the flowing blood sample, wherein the second evanescent field extends into the plasma layer of the flowing whole blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the flowing whole blood sample containing blood cells; and 
 measuring absorption of the light from second light source by the analyte in only the plasma layer of the flowing whole blood sample within the second evanescent field. 
 
     
     
       2. The method of  claim 1 , comprising:
 receiving the flowing whole blood sample in a channel, wherein the optically transmissive medium adjacent to the channel includes the first surface abutting the channel, the first surface defining the optical interface between the optically transmissive medium and the flowing whole blood sample. 
 
     
     
       3. The method of  claim 1 , comprising:
 aiming a first light detector to receive the light from the first light source that has been reflected through the optically transmissive medium from the optical interface. 
 
     
     
       4. The method of  claim 1 , comprising identifying a presence of analytes in the evanescent field by comparing an intensity of the light from the first light source that has been reflected through the optically transmissive media at a first wavelength with a predetermined intensity. 
     
     
       5. The method of  claim 4 , wherein the predetermined intensity is an intensity of light emitted from the first light source. 
     
     
       6. The method of  claim 1 , comprising: comparing the absorption of light from the second light source by the analyte to the absorption of light from the first light source by the analyte to determine an absorption difference; and
 detecting the analyte in the flowing whole blood sample based on the absorption difference. 
 
     
     
       7. A method for detecting analytes in whole blood without red blood cell separation from the whole blood, the method comprising:
 receiving a whole blood sample in a channel, wherein an optically transmissive medium adjacent to the channel includes a first surface abutting the channel, the first surface defining an optical interface between the optically transmissive medium, wherein the optically transmissive medium is glass and the blood sample when the whole blood sample is received in the channel; 
 directing a first light source through the optically transmissive medium to the optical interface at an angle of incidence greater than or equal to a critical angle relative to a normal of the optical interface, wherein the angle of incidence creates total internal reflection of light from the first light source and creates an evanescent field extending into the channel, wherein the evanescent field extends into a plasma layer of the blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the channel containing blood cells; 
 aiming a first light detector to receive the light from the first light source that has been reflected through the optically transmissive medium from the optical interface; 
 measuring absorption of the light from the first light source by an analyte in only a plasma layer of the whole blood sample within range of the evanescent field; and 
 directing a second light source through optically transmissive medium to the optical interface, wherein light from the second light source has a wavelength that differs from a wavelength of the light from the first light source and wherein the light from the second light source is incident at the optical interface at second an angle of incidence greater than or equal to the critical angle relative to the normal of the optical interface, wherein the second angle of incidence creates total internal reflection of the light from the second light source and creates a second evanescent field extending into the flowing blood sample, wherein the second evanescent field extends into the plasma layer of the flowing whole blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the flowing whole blood sample containing blood cells; and 
 measuring absorption of the light from second light source by the analyte in only the plasma layer of the flowing whole blood sample within the second evanescent field. 
 
     
     
       8. An apparatus for determining an analyte content in blood, the apparatus comprising:
 a channel for receiving the flowing blood sample; 
 an optical interface between a flowing blood sample and an optically transmissive media, wherein the optically transmissive media is glass, wherein the optically transmissive medium includes a first surface abutting the channel, the first surface defining an optical interface between the optically transmissive medium and the following blood sample; 
 a first light source directed through the optically transmissive medium to the optical interface at an angle of incidence greater than or equal to a critical angle relative to a normal of the interface, wherein the angle of incidence creates total internal reflection of light from the first light source and creates the evanescent field extending into the channel, wherein the evanescent field extends into a plasma layer of the flowing blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the channel containing blood cells; 
 a first light detector configured to detect absorption of light from the first light source in the evanescent field at a first wavelength corresponding to an absorption wavelength of the analyte; 
 a second light source directed through optically transmissive medium to the optical interface, wherein light from the second light source has a second wavelength that differs from a first wavelength of the light from the first light source and wherein the light from the second light source is incident at the optical interface at a second an angle of incidence greater than or equal to the critical angle relative to the normal of the optical interface, wherein the second angle of incidence creates total internal reflection of the light from the second light source and creates a second evanescent field extending into the flowing blood sample, wherein the second evanescent field extends into the plasma layer of the flowing whole blood sample adjacent to the optical interface and decays to substantially zero before reaching a portion of the flowing whole blood sample containing blood cells; and 
 a second light detector configured to detect absorption of light from the second light source in the evanescent field at the second wavelength corresponding to the absorption wavelength of the analyte. 
 
     
     
       9. The apparatus of  claim 8 , wherein the optically transmissive media comprises a prism. 
     
     
       10. The apparatus of  claim 9 , wherein the first light source comprise a light emitting diode. 
     
     
       11. The apparatus of  claim 9 , wherein the first light source comprises a laser diode. 
     
     
       12. The apparatus of  claim 9 , wherein the first light source is configured to direct light onto the optical interface at a wavelength corresponding to the absorption wavelength of the analyte, and wherein the angle of incidence is selected to provide total internal reflection of the light within the optically transmissive media. 
     
     
       13. The apparatus of  claim 8 , wherein the first light detector aimed to receive the light from the first light source that has been reflected through the optically transmissive medium from the optical interface. 
     
     
       14. The apparatus of  claim 13 , wherein the light detector comprises a photo-diode. 
     
     
       15. The apparatus of  claim 8 , wherein the channel is configured to generate a cell free layer of the of the flowing blood at a the optical interface of the channel. 
     
     
       16. The apparatus of  claim 8 , wherein the analyte comprises free hemoglobin in the cell free layer of the flowing blood. 
     
     
       17. The apparatus of  claim 16 , wherein the light emitted by the light emitter has a wavelength of between 410 nanometers and 420 nanometers. 
     
     
       18. The apparatus of  claim 17 , wherein the light detector is configured to detect light absorption at wavelengths between 410 nanometers and 420 nanometers.